This invention relates to methods and apparatus for optically inspecting workpieces such as substrates used to make magnetic disks or magnetic disks during any point in the manufacturing process (including the finished disk).
Magnetic disks are typically manufactured using the following method.    1. A disk-shaped substrate (typically an Al alloy) is lapped or ground.    2. A material such as a nickel phosphorus alloy is plated onto the substrate.    3. The plated substrate is polished and textured. (During texturing, texture grooves are typically formed in the substrate by mechanical abrasion to cause a subsequently deposited magnetic layer to exhibit anisotropy. It is also known to laser texture substrates for tribological reasons.)    4. One or more underlayers, one or more magnetic layers and one or more protective overcoats are deposited onto the plated substrate. (The deposition process can comprise sputtering or other techniques.) Other layers can also be deposited onto the substrate during manufacturing.    5. A lubricant is applied to the disk.
At various points during manufacturing (e.g. before or after texturing), it is desirable to inspect the substrate for bumps, pits, contaminant particles, or other defects. During such inspection, one should be able to detect very small defects. It is known in the art to use lasers to scan such substrates for this purpose. See, for example, U.S. Pat. Nos. 6,566,674 and 6,548,821, issued to Treves et al. (The Treves patents are incorporated herein by reference.)
We have developed an improved optical test apparatus comprising upper and lower test heads for applying upper and lower laser beams to the upper and lower workpiece surfaces. The upper and lower test heads also include one or more detectors for receiving light reflected off of the upper and lower workpiece surfaces.
It is desirable to prevent or reduce “cross-talk” between the upper and lower test heads during testing.